Device for thermal insulation of at least a submarine pipeline comprising a phase-change material confined in jackets

ABSTRACT

A device for thermally insulating at least one undersea pipe. The device comprises a thermally insulating covering surrounding the pipe, the covering being covered by an outer leak proof protective case, and the case being made of a flexible or semi rigid material suitable for remaining in contact with the outside surface of the insulating covering when it deforms. The device is characterized in that the insulating covering comprises a phase-change material confined in at least one container made of a flexible or semi rigid material that is deformable, and the container is disposed around the pipe.

The present invention relates to devices and method of thermallyinsulating at least one undersea pipe resting on the sea bottom, inparticular at great depths, for connecting the sea bottom to anchoredinstallations floating on the surface.

The invention relates more particularly to pipes connecting the seabottom to anchored installations floating on the surface.

The technical field of the invention is that of making and assemblinginsulating systems outside and around pipes for conveying hot effluentsfrom which it is desired to limit losses of heat.

The invention applies more particularly to developing oil fields in deepseas, that is to say offshore oil installations where surface equipmentis generally situated on floating structures while the wellheads are atthe bottom of the sea. The pipes concerned by the present invention aremore particularly risers, i.e. pipes providing a bottom-to-surfaceconnection by rising towards the surface, however the invention alsoapplies to pipes connecting wellheads to said riser pipes.

Present developments in deep seas are generally performed in depths ofwater that can be as great as 1500 meters (m). Future developments areanticipated in depths of water of as much as 3000 m to 4000 m, and evenmore.

The main application of the invention is thermally insulatingunderwater, sub-sea, or immersed pipes or ducts, and more particularlythose at great depths, more than 300 m, serving to convey hot petroleumproducts which will lead to difficulties if they cool excessively,whether under normal production conditions or in the event of productionbeing stopped.

In that type of application, numerous problems arise if the temperatureof the petroleum products decreases by a significant amount comparedwith their production temperature, which production temperature isgenerally in the range 60° C. to 80° C., or even higher, while thetemperature of the surrounding water, particularly at great depths, canbe well below 10° C., and can be as little as 4° C. If the petroleumproducts cool to below a certain temperature T₁, which depends on thequality of the products concerned, where the temperature T₁ generallylies in the range 20° C. to 60° C., for example, then the following areobserved:

-   -   a great increase in viscosity, which reduces the flow rate in        the pipe;    -   a precipitation of dissolved paraffin which then increases the        viscosity of the product, and by being deposited can reduce the        effective inside diameter of the pipe;    -   flocculation of asphaltenes, leading to the same problems; and    -   the sudden, compact, and massive formation of gas hydrates which        precipitate at high pressure and low temperature, thus suddenly        obstructing the pipe.

Paraffins and asphaltenes remain stuck to the wall which must then becleaned by scraping the inside of the pipe; in contrast hydrates areeven more difficult and sometimes even impossible to resorb.

One of the functions of thermally insulating such pipes is thus to slowdown the cooling of the petroleum effluent conveyed so that itstemperature does not drop below T₁, e.g. 40° C. on reaching the surface,for a production temperature at the inlet of the pipe of T₂=60° C. to80° C., not only under steady production conditions, but also in theevent of the rate of production decreasing or even stopping, in order toensure that the temperature of the effluent does not drop too far belowthe temperature T₁, e.g. below 30° C., in order to limit theabove-mentioned problems, or at least to ensure that they remainreversible.

With the installation of single pipes or of bundles of pipes, it isgenerally preferred to prefabricate the pipes on land in unit lengths of250 m to 500 m, which lengths are then towed offshore by a tug. For atower type bottom-to-surface connection, the length of the pipegenerally constitutes 50% to 95% the depth of the water, i.e. it can beas much as 2400 m for a depth of 2500 m. While it is being built onland, the first unit length is pulled out to sea and the followinglength is connected to its end, the tug keeping the assembly undertraction throughout the end-to-end joining stage which can last forseveral hours, or even several days. Once the entire pipe or bundle ofpipes is in the water, it is towed to the site, generally below thesurface in a substantially horizontal configuration, where it is then“up-ended”, i.e. tilted vertically so as to reach the vertical position,after which it is put in its final position.

A device for thermally insulating at least one undersea pipe is knownthat comprises an insulating outer covering surrounding the pipe, and anouter protective case, said outer case performing two functions:

-   -   firstly preventing damage which could arise during manufacture        and towing, and also during laying, particularly in shallow        zones, where towing can under some circumstances take place over        distances of several hundred kilometers; for this purpose, the        materials used are quite strong, such as steel, a thermoplastic        or thermosetting compound, or indeed a composite material; and    -   secondly creating a leakproof confinement around the insulating        system; this confinement is necessary with insulating outer        coverings constituted by materials that can be subject to        migration, or indeed that include fluid compounds.

In depths of 2000 m, hydrostatic pressure is about 200 bars, i.e. 20megapascals (MPa), which implies that the set of pipes and theircovering of insulating material must be capable of withstanding not onlysuch pressures without damage during pressurization and depressurizationof the pipe in which the hot fluid flows, but must also be capable ofwithstanding temperature cycles that lead to changes in the volumes ofthe various components, and thus to positive or negative pressures thatcan lead to partial or total destruction of the case, either byexceeding acceptable stresses, or by implosion of the outer case(pressure variants leading to negative internal pressures).

Document WO 00/40886 discloses an insulating device in which asolid-liquid insulating phase-change material is used having a latentheat of fusion with a phase change that takes place at a temperature T₀that is higher than the temperature T₁ at which the petroleum flowinginside the pipe becomes too viscous, where the temperature T₁ generallylies in the range 20° C. to 60° C. and is lower than the temperature T₂of the crude oil penetrating into the pipe.

In the event of production stopping, a phase-change material (PCM) makesit possible to conserve the fluid that would normally be flowing insidethe inner pipe at a temperature that is high enough to avoid paraffinsor hydrates forming in the petroleum product.

Thus, in the event of production stopping, the crude oil ceases to flowand remains in position within the pipe, and the loss of heat to theexternal environment, generally at 4° C. in very great depths, takesplace to the detriment of the PCM, the crude oil continuing to remain ata temperature that is greater than or substantially equal to the phasechange temperature of said PCM.

Throughout the solidification or crystallization of the PCM, thetemperature of the PCM remains substantially constant and equal to T₀,e.g. 36° C., and thus the inner pipe containing the crude oil remains ata temperature that is greater than or substantially equal to thetemperature (T₀) of the PCM, i.e. 36° C., thus preventing paraffins orhydrates forming in the crude oil.

Said insulating phase-change material is preferably selected for its lowthermal conductivity, and in particular conductivity of less than 0.5watts per meter per degree Celsius (W/m/K).

Said PCM insulating material is selected in particular from materialsconstituted by at least 90% chemical compounds selected from alkanes, inparticular having a hydrocarbon chain of at least 10 carbon atoms, orindeed optionally hydrated salts, glycols, bitumens, tars, waxes, andother fatty materials that are solid at ambient temperature, such astallow, margarine, or fatty alcohols and acids, and preferably theincompressible material is constituted by paraffin having a hydrocarbonchain of at least 14 carbon atoms.

The phase-change materials described in the past generally presentsignificant change in volume on changing state, this change in volumepossibly being as much as 20% for paraffins. The outer protective casemust be capable of accommodating such variations in volume withoutdamage.

That is why, according to WO 00/40886, the insulating phase-changematerial is confined within a leakproof and deformable case that is thuscapable of following the expansion and the contraction of the variouscomponents under the influence of all of the surrounding parameters, andin particular internal and external temperatures. The pipe is thusconfined within a semirigid or flexible thermoplastic case, inparticular one made of polyethylene or polypropylene, and one that iscircularly shaped, for example, with any increase or reduction in itsinside volume due to temperature variations being comparable tobreathing and being absorbed by the flexibility of the case which isconstituted, for example, by a thermoplastic material presenting a highelastic limit. However, in order to withstand mechanical stresses, it ispreferable to use a case that is semirigid, being made of a strongmaterial such as steel or a composite material, for example a compoundbased on a binder such as epoxy resin and organic or inorganic fiberssuch as glass fibers or carbon fibers, but under such circumstances thecase is given an oval or flattened shape with or without reentrantportions so as to give it a section that for given perimeter is lessthan that of the corresponding circle. Thus, the “breathing” of thebundle will lead, in the event of an increase and a decrease in volume,respectively to the case being made rounder and to the case being madeflatter. Under such circumstances, the case and bundle assembly isreferred to as a “flat bundle” in contrast to a circular case.

In WO 00/40886, the PCM is absorbed within an absorbent matrix, and itoccupies all of the space that exists between the pipe and said outercase with which it always remains in contact, said case beingdeformable.

While an insulating device of that invention is being made, whichpreferably takes place on land, the space between the pipe(s) and theouter case is filled with the PCM while in the liquid state, i.e. whilehot. Nevertheless, there is a risk during filling with said PCM thatsaid PCM might solidify locally, thus preventing the volume from beingfilled completely. Under such circumstances, empty zones or gas pocketsare created that are harmful firstly to the insulating effect duringfuture operation of the installation, and secondly and above all tooverall strength since there is a risk of the case collapsing locallywhen the pipe is installed in great depths, i.e. when it is subjected tovery high hydrostatic pressures. These problems are easily overcome onshort lengths of pipe, for example 6 m or 12 m, but they are much moredifficult to avoid over significant lengths, for example more than 100m.

In order to overcome those drawbacks, techniques have been developed inthe prior art that are based on insulating devices comprising aninsulating material constituted by a gel that presents a high degree ofinsulation. Gelling presents the advantage of avoiding convectionphenomena within the insulating mass. In addition, the gel is generallyobtained by physical, chemical, or physico-chemical reactions betweenvarious components, thus enabling the gel to be injected in liquid formimmediately after its components have been mixed together, it beingpossible to fill the case completely before bulk gelling begins insignificant manner.

Embodiments have thus been proposed in which the insulating PCM isformulated in the form of particles or microcapsules of said PCM thatare uniformly dispersed within a matrix of a primary insulatingmaterial, in particular an insulating gel in order to make it easier tooccupy the entire space between the pipe and the outer case.

Nevertheless, that technique presents the drawback of the quantity ofPCM surrounding the pipe being necessarily reduced since it isdistributed discontinuously around the pipe.

In addition, the inventors have found that it is only the PCM that isclose to the hot pipe that can accumulate heat on liquefying, since thePCM close to the outer case is generally at the temperature of the seabottom, i.e. 4° C., and thus does not contribute to the process ofaccumulating heat, i.e. it remains permanently in the solid orcrystallized state. This fraction of the PCM close to the outer case isthus ineffective and useless and can even be harmful if the PCM usedpresents high intrinsic conductivity, as is the case for metallic salts.

Prior embodiments have also been described for applications in which thepipe rests horizontally on the sea bottom. However certain problems thenarise for bottom-to-surface connections.

With a bottom-to-surface connection, for example the vertical portion ofa tower, or indeed a catenary section connecting the top of the tower toa support on the surface, or also with pipes resting on a deep slope onthe sea bottom, the external pressure varies along the pipe, decreasingon rising towards the surface. With insulating materials that are inpaste or fluid form, such as PCMs, the material presents density that isless than that of sea water, generally having a relative density lyingin the range 0.8 to 0.85, so a pressure differential between the insideand the outside will vary along said pipe, increasing on approaching thesurface. Thus, greatest deformation occurs in the portions that presentthe greatest pressure differential, thereby leading to importanttransfers of fluid parallel to the longitudinal axis of said pipe. Inaddition, such transfers are amplified by the “breathing” phenomena dueto temperature variations, as described above.

A “flat bundle” is sensitive to pressure variations due to slopes:higher pressure lower down, lower pressure higher up, and the towingstage is critical since the length of the bundle can be as much asseveral kilometers, and the bundle is never accurately horizontal,giving rise to significant pressure variations during said towing, andabove all during the up-ending operation for a bottom-to-surfaceconnection.

When the bundle is in the vertical position or on the sea bottom on asignificant slope, the pressure differential created by the low densityof the insulating material, associated with the variation in volumecreated by the thermal expansion of the insulating material, leads tomovements in the insulating material that the outer case must be capableof withstanding. It is desirable to avoid particles moving parallel tothe axis of the bundle, i.e. migration of insulating material betweentwo remote zones of the bundle, since that runs the risk of destroyingthe physical structure proper of the insulating material.

In order to ensure that the bundle behaves well throughout its lifetime,it is desirable for it not to contain any residual gas. With aninsulating mixture that is pasty or semifluid, any pocket of gas thatresults from the manufacturing process will have repercussions, firstlyduring transport since once the bundle is being towed at significantdepth, the ambient pressure will compress the residual gas, which runsthe risk of significantly reducing its buoyancy and can lead tosituations that are dangerous not only for the equipment but also forpersonnel; and secondly, while it is being put into a vertical position,any pockets of compressed gas will tend to come together towards the topof the bundle, thus running the risk of creating a significant length ofpipe that does not have any insulating compound.

A first object of the present invention is thus to provide a device forthermally insulating at least one undersea pipe including a phase-changematerial, preferably an insulating phase-change material that satisfiesthe following targets:

-   -   a) the insulating device is easy to manufacture, in particular        in terms of filling said insulating phase-change material in        such a manner that eliminates, or at least reduces, any risk of        creating voids or pockets of gas in the volume to be filled        between the pipe and the outer case; and    -   b) the phase-change material is localized and distributed around        the pipe and in the vicinity of the pipe rather than in the        vicinity of the case, so as to improve its effectiveness.

Another object of the present invention is to provide a system forinsulating a pipe that includes a covering of PCM of semifluid or pastytype that can be put into place around the pipe, on land, more simplyand at lower cost, in such a manner as to ensure that the pipe can betowed under the surface and up-ended into a vertical position in orderto be installed, while ensuring that the entire assembly remainsundamaged until it is put into production and throughout its workinglifetime, which generally exceeds 30 years.

Another object is to be able to insulate at least one undersea pipe forbottom-to-surface connection or for laying on the bottom, in particularat great depth, and in particular in a steeply sloping zone, that iscapable of providing significant transverse flexibility in order toabsorb variations in the volume of the insulating PCM contained in anouter case surrounding the pipe, while nevertheless conservingsufficient longitudinal rigidity to make handling possible, both duringmanufacture on land, and during towing to the site, and for conservingmechanical integrity of said case throughout the lifetime of the pipe,which can exceed 30 years.

One problem posed is thus to minimize longitudinal migration ofinsulating materials subject to migration and contained inside saidcase, with this being particularly significant when said insulatingmaterial is semifluid or pasty, and in particular of the type comprisinga gelled insulating matrix, given the risk of the performance of theinsulating mixture deteriorating in the event of excess internal shearbeing applied to said insulating matrix.

In addition, another problem is to create a system for insulating anundersea pipe or a bundle of pipes organized as a “flat” bundlecontaining a phase-change material, preferably an insulating PCM, andhaving phase behavior on restarting that is such that restarting can beperformed in controllable time, and in particular in time that isshorter than has been possible in the prior art.

In the event of a stoppage lasting several days or even several weeks,while the PCM is still active, precautions are generally taken to purgethe line by causing a substitute substance to flow in a loop, e.g. gasoil, so as to keep the assembly in a safe condition prior to allowingthe temperature of the pipe to drop down to 4° C. Thereafter, onrestarting, it is general practice to use the same gas oil so as toreheat the pipe by causing it to flow in a loop from the floatingsupport where it is heated by being passed through boilers or throughheat exchangers that recycle heat taken from gas turbines.

Thus, during heating, heat migrates from the inside of the pipe towardsthe outer surrounding medium which is generally at 4° C., and throughoutthe heating stage, the major fraction of the heat conveyed by thecirculating gas oil is absorbed by the PCM so as to reliquefy it, withthis stage possibly lasting several days or even several weeks if thepipe is very long, or if the amount of heat generated on the floatingsupport is not sufficient. It is only after this stage of heating thepipe by circulating gas oil that it is possible to reconnect thewellheads and restart production.

If production is restarted prematurely, the PCM will only be partiallyliquid and its internal temperature will be less than or equal to T₀(the phase change temperature), and thus low over the undersea pipe as awhole, in which case the following phenomena can be observed.

As the petroleum leaving the well at high temperature, e.g. 75° C.advances towards the floating production storage and off-loading (FPSO)unit, it delivers heat to the PCM for liquefying it, and as a result thetemperature of the petroleum drops quickly since the PCM then acts notas an insulating system but inversely as an absorber of heat, thusleading to accelerated cooling of the crude oil. As a result, aftertraveling a few kilometers, or even after traveling only a few hundredsof meters, the temperature of the oil drops below the critical value T₁at which the unwanted phenomena of hydrate or paraffin plug formationoccur within the oil flowing along the pipe, and that can lead to thestream of crude oil being blocked. In the zone close to the wellhead,the PCM will reliquefy progressively and the reliquefication front willadvance slowly towards the FPSO. In a more remote zone, the temperaturewill remain stable at around T₀ and liquefication cannot continue unlessthe crude oil is still at a temperature higher than T₀.

Thus, with very long lines, e.g. 5 km or 6 km long, in a zone that isvery far from the hot source, i.e. close to the FPSO, there will not beenough heat delivered and the PCM will lose heat to the ambient mediumat 4° C. To provide this heat, it will change progressively to the solidstate.

Thus, particularly for very long pipes, it can be seen that whenrestarting the PCM in the zone close to the wellheads can be in areliquefication stage, whereas at the other end, close to the FPSO, thePCM is in a resolidification stage, since the rate at which heat isbeing lost to the ambient medium is greater than the rate at which heatis being supplied by the crude oil flowing in the pipe. The PCM iswaiting for a hot front of crude oil to transform it back into theliquid phase.

Finally, a last object of the present invention is thus to provide asystem for insulating a pipe, the system comprising an insulatingphase-change medium which does not absorb heat too quickly from the hotfluid flowing in the pipe during a restarting stage, thus making itpossible to lengthen the period during which the temperature of thefluid flowing in an undersea pipe can be maintained above a fixed value,such that the length of time required for heating by means of asubstitution substance circulating in a loop, after a prolonged stoppageis shortened and thus ensuring that the duration of the restarting stageis shortened, so that, for example, it can suffice to heat the pipe inpart only without it being necessary to wait for all of the PCM to becompleted liquefied.

To do this, the present invention provides a device for thermallyinsulating at least one undersea pipe, the device comprising:

-   -   a thermally insulating covering surrounding said pipe(s);    -   said covering itself being covered by an outer leakproof        protective case, and said case being made of a flexible or        semirigid material suitable for remaining in contact with the        outside surface of said insulating covering when it deforms,    -   the device being characterized in that:    -   said insulating covering comprises a phase-change material,        preferably an insulating phase-change material confined in at        least one container made of a flexible or semirigid material        that is deformable; and    -   said container(s) being disposed around said pipe(s).

The inventors have found that confining the phase-change material inpackages in the form of deformable flexible containers presents numerousadvantages, and serves to solve the above-mentioned problems anddrawbacks, namely:

-   -   1) it is much easier to put the PCM into place around the pipe        since said containers are filled with the PCM separately and in        advance;    -   2) the PCM can be localized in the zone where it is most        effective, i.e. essentially localized close to the pipe;    -   3) the containers contribute to maintaining said insulating        phase-change material in shape since it is subject to migration.        However since said containers are flexible or semirigid, and        preferably semirigid, the phase-change material can be subject        to changes in volume as a result of changing phase since said        containers can deform; and    -   4) the containers make it possible to use insulating PCMs inside        the case and also to use other insulating materials outside the        case that would be chemically incompatible therewith were they        to come into contact.

A container is said to be “semirigid” herein when it is sufficientlyrigid to keep its shape in spite of the weight and the pressure exertedby the phase-change material it contains, while nevertheless presentingsufficient flexibility to accept deformations, and in particular tofollow the deformations that result from the material changing volumewhen it changes phase.

A preferred embodiment is characterized in that in a cross-section ofsaid pipe(s), level with said container(s), said pipe(s) is/aresurrounded by said container(s) in substantially continuous manner.

This means that said container(s) is/are distributed and assembledaround each of said pipes taken separately, or around the set comprisingsaid two pipes, in a manner that is substantially continuous, so thateach pipe is thus practically entirely separated from said outerprotective case by said containers. This makes it possible to ensurethat the insulation around each pipe or each of said pipes is uniform,thereby avoiding the appearance of cold points.

More particularly, in the portions of the pipe(s) surrounded by saidcontainers, the device comprises at least two and preferably three orfour containers in a cross-section of said pipe(s) surrounded by saidcontainers, and preferably surrounding said pipe(s) in a manner that issubstantially continuous.

In another embodiment, the pipe(s) is/are surrounded by a singlecontainer folded onto itself in such a manner that its longitudinaledges are situated facing each other at a small distance apart.

Also preferably, said containers are placed close to the pipe in such amanner that the pipe does not come directly into contact with at leastsome of said containers, and preferably not with any of said containers.

The space between the PCM and the pipe makes it possible to reduce therate at which heat is absorbed from oil leaving the wellhead during astage of restarting production after a stoppage, thus enabling the oilto remain at temperature for as long as possible and reach thetemperature T₀ only when it is close enough to the surface to be able toreach the surface without its temperature dropping below the temperatureT₁ at which some of its components freeze, thus preventing flow withinthe pipe.

In an embodiment, said containers are placed against spacers, saidspacers being placed against and around said pipe so as to leave a gapbetween said containers and said pipe.

It is advantageous for the distance between the pipe and a container notto be excessive, so as to reduce the quantity of PCM that needs to beimplemented.

More particularly, said containers should be spaced apart from said pipeat a distance lying in the range 5 millimeters (mm) to 10 centimeters(cm), and preferably in the range 1 cm to 5 cm.

In another advantageous embodiment, said pipe is surrounded by a secondinsulating material that is solid and pressed against said pipe,preferably in the form of a shell of syntactic foam, with saidcontainers being pressed against said solid insulating materialsurrounding said pipe.

This configuration serves to further slow down the transfer of heat fromthe fluid flowing in the pipe to the insulating PCM, while restartingproduction inside the pipe.

In a preferred embodiment, said insulating covering covered in a saidleakproof protective case comprises a main insulating material,preferably an insulating gel, placed between said outer case and saidcontainer(s) of insulating phase-change material surrounding saidpipe(s).

In an embodiment, said main insulating material also constitutes aphase-change material which can be identical to or different from saidoptionally insulating phase-change material contained in saidcontainers.

In an embodiment, said main insulating material surrounds said pipe(s)and separates said pipe(s) from said containers in the gap between saidpipe(s) and said container(s).

As mentioned above, said phase-change material presents a liquid/solidmelting temperature T₀ that preferably lies in the range 20° C. to 80°C., that is higher than the temperature T₁ at which the fluid flowinginside the pipe presents an increase in viscosity that is harmful forits ability to flow in said pipe, and lower than the temperature T₂ ofthe fluid flowing in the pipe when it is in operation.

The term “insulating” material is used herein to mean a material thatpreferably presents thermal conductivity of less than 0.5 W/m/K, andmore preferably lies in the range 0.05 W/m/K to 0.2 W/m/K.

More particularly, said insulating phase-change material compriseschemical compounds from the alkane family, preferably a paraffin havinga hydrocarbon chain with at least 14 carbon atoms.

Still more particularly, said paraffin is heptacosane having the formulaC₁₇H₃₆, and presenting a melting temperature of abut 50° C.

It is also possible to use an industrial paraffin cut centered onheptacosane.

In an embodiment, said main insulating material situated in particularbetween said containers and said outer case is constituted by aninsulating mixture comprising a first compound consisting in ahydrocarbon compound such as paraffin or gas oil, mixed with a secondcompound consisting in a gelling compound and/or a compound having astructuring effect, in particular by cross-linking, such as a secondcompound of the polyurethane type, of the cross-linked polypropylenetype, of the cross-linked polyethylene type, or of the silicone type,and preferably said first compound is in the form of particles ormicrocapsules dispersed within a matrix of said second compound.

As first compounds, more particular mention can be made of chemicalcompounds of the alkane family such as paraffins or waxes, bitumens,tars, fatty alcohols, glycols, and still more particularly compoundshaving a melting temperature of the materials lying in the range betweenthe temperature T₁ of the hot effluent flowing in one of the pipes andthe temperature T₂ of the medium surrounding the pipe in operation,i.e., in fact and in general, a melting temperature lying in the range20° C. to 80° C. For example, it is possible to use heptacosane havingthe formula C₁₇H₃₆ as the paraffin, which presents a melting temperatureof about 50° C.

These various insulating materials are materials that are “subject tomigration”, i.e. materials that are liquid, pasty, or of a solidconsistency such as the consistency of a grease, a paraffin, or a gel,which can be deformed by the stresses that result from differentialpressures between two distinct points of the case and/or because oftemperature variations within said insulating material.

That is why, according to another characteristic of the presentinvention, a device for thermally insulating at least one undersea pipehas at least two leakproof transverse partitions, each of saidpartitions being constituted by a closed rigid structure through whichsaid pipe(s) pass(es), and secured to said pipe(s) and to said case,with said containers being disposed around said pipe(s) between said twotransverse partitions.

This rigid structure secured to the case prevents said case moving pastsaid partition and relative thereto, thus freezing the geometry of thecross-section of the case and said partition.

The terms “leaktight” and “closed” mean that said partition does notallow the material constituting said insulating covering from passingthrough said partition, and in particular that the junction between saidpipe and the orifices through which the pipe passes through saidpartition does not allow said insulating covering material to passthrough.

Said leaktight partitions serve to confine said insulating material(s)that are subject to migration and that constitute said insulatingcovering between said case and said partitions.

The term “cross-section” is used to mean the section in a plane XX′, YY′perpendicular to the longitudinal axis ZZ′ of said case, said case beingtubular in shape and presenting a central longitudinal axis ZZ′, and thecross-section of said case preferably defines a perimeter presenting twomutually perpendicular axes of symmetry XX′ and YY′ that areperpendicular to the longitudinal axis ZZ′.

In a particular embodiment, said closed structure of said leaktighttransverse partition comprises a cylindrical piece presenting across-section whose perimeter presents the same fixed shape as that ofthe cross-section of the case.

In the present description, the term “perimeter” of a cross-section isused to mean the line in the form of a closed curve which outlines theplane surface defined by said cross-section.

The perimeter of the cross-section of the case at the leaktightpartitions is of fixed shape and therefore cannot deform by contractionor expansion of said case at that point.

In different variant embodiments, said cross-section of the case iscircular in shape, or oval in shape, or indeed rectangular in shape,preferably having rounded corners.

When the device has at least two pipes disposed in a common plane, thecross-section of said case is preferably elongate in the same directionas said plane.

More particularly, the outer perimeter of the cross-section of saidprotective case is a closed curve for which the ratio of the square ofthe length over the area it defines is not less than 13, as described inWO 00/04886.

During variations in the internal volume, the case tends to deformtowards a circular shape which, mathematically, constitutes the shapethat presents the largest area for given perimeter.

For a leakproof case of circular profile, an increase in volume leads tostresses in the wall, which stresses are associated with an increase inpressure that results from this increase in volume.

In contrast, if the shape of the cross-section of the outer covering isflattened, the ability of its case for absorbing expansion due to thevarious compounds expanding under the effect of temperature, withoutcreating significant extra pressure, is improved since the case canbecome rounder.

If the profile is oval in shape, a variation in internal pressure willlead to a combination of bending stresses and pure traction stressessince the varying curvature of the oval then behaves like anarchitectural vault, but with the difference that under suchcircumstances the stresses are traction stresses and not compressionstresses. Thus, a shape that is oval or close to oval can be envisagedfor small expansion capacity, and under such circumstances it isappropriate to consider oval shapes having a ratio of major axis ρmaxover minor axis ρmin that is as high as possible, for example at least2/1 or 3/1.

The shape of the case is then selected as a function of the totalexpansion of the volume of the outer insulating coating under the effectof temperature variations. Thus, for an insulating system mainly usingmaterials that are subject to expansion, a rectangular shape, apolygonal shape, or indeed an oval shape can enable the wall to expandby bending while minimizing the traction stresses within the outer case.

For an insulating material presenting a high degree of expansion underthe effect of temperature variations, such as gas oil, substances in thealkane family (paraffin), or indeed phase-change materials, therectangle should be flatter so as to create the necessary expansionreserve. This expansion reserve can be further increased by creatingreentrant curves, in known manner.

Said partitions create thermal bridges. It is therefore desirable tospace them apart as much as possible in order to reduce the thermalbridges.

In a particular embodiment, the spacing between two such successiveleaktight partitions along the longitudinal axis ZZ′ of said case liesin the range 50 m to 200 m, and in particular in the range 100 m to 150m.

In order to reduce the number of leaktight partitions, according to apreferred characteristic, a device of the invention comprises one, andpreferably a plurality, of shaping templates disposed transversely tosaid longitudinal axis ZZ′, each constituted by an open rigid structuresecured to said pipe(s) which pass through it, and secured to said caseat its periphery, being disposed between two of said successiveleaktight partitions, and preferably at regular intervals along saidlongitudinal axis ZZ′, said shaping template preferably presentingopenings allowing the material constituting said main insulatingmaterial to pass said shaping template.

Like said leaktight partition, said shaping template freezes the shapeof the cross-section of the case at said shaping template, whileminimizing heat bridges.

More particularly, said open structure of said shaping templatecomprises a cylindrical part of a cross-section whose perimeter isinscribed within a geometrical figure identical to the geometricalfigure defined by the shape of the perimeter of the cross-section ofsaid leaktight partition.

Preferably, the device of the invention has a plurality of shapingtemplates disposed along said longitudinal axis ZZ′ of the case,preferably at regular intervals, with two successive shaping templatesbeing spaced apart preferably by 5 m to 50 m, and more preferably by 5 mto 20 m.

In a preferred embodiment, the device has at least one centralizingtemplate disposed transversely to said longitudinal axis ZZ′, preferablyat regular intervals, between two successive ones of said leaktightpartitions and/or between two of said shaping templates along saidlongitudinal axis ZZ′, each centralizing template being constituted by arigid part secured to the pipe(s) and presenting a shape which allowslimited movement of said case in contraction and in expansion level withsaid centralizing template, said containers being disposed between twosuccessive centralizing templates, where appropriate.

The centralizing template seeks to ensure that there is some minimumcovering of insulating covering (3) around said pipe(s) in the event ofthe case deforming by contraction and transferring said displaceablematerial between said two leaktight partitions and/or between saidshaping templates.

More particularly, said centralizing template is constituted by a rigidpart, preferably having a cylindrical outside surface, with across-section whose perimeter is set back from that of said leaktightpartition, serving to limit deformation of said case by the case comingdirectly into mechanical abutment on said rigid part at at least twoopposite points of the perimeter of the cross-section of said case.

Said centralizing template presents a cross-section whose perimeter isinscribed within a geometrical figure which is substantiallygeometrically similar to the geometrical figure defined by the perimeterof the cross-section of said leaktight partition.

In an embodiment, said rigid part constituting said centralizingtemplates presents a portion of its outside surface that is set backsufficiently from the surface of the case, and/or presents throughperforations, to establish a gap which allows the material constitutingsaid main insulating material to be transferred through saidcentralizing template.

The distance between two centralizing templates along said longitudinalaxis ZZ′ is such as to enable it to maintain a quantity of the materialcomprising said insulating coating that is sufficient to provide aminimum covering needed for thermally insulating said pipe, given thecontraction deformation to which said case can be subjected.

Advantageously, the device of the invention has a plurality ofcentralizing templates, with two successive centralizing templates beingspaced along said longitudinal axis ZZ′ of the case by a distance of 2 mto 5 m, with said containers being interposed between two successivecentralizing templates.

More particularly, the present invention provides a device of theinvention including at least two of said underwater pipes placed inparallel.

Under such circumstances, and advantageously, said leaktight partitions,shaping templates, and centralizing templates serve to keep said atleast two undersea pipes at a fixed distance apart.

The present invention also provides a unit thermally insulating devicesuitable for use in building up a device of the invention by assemblingsaid unit thermally insulating devices end to end, the unit device beingcharacterized in that it comprises:

-   -   one or more unit undersea pipe elements occupying the place of        the undersea pipe(s); and    -   a said protective case and a said covering comprising a said        container containing a said phase-change material, and, as        defined above, each said unit element comprising at at least one        of its ends or at each of its ends a said leaktight partition,        and preferably said centralizing templates, and more preferably        said shaping templates as defined above, disposed between two        successive leaktight partitions.

The invention also provides a method of assembling a unit device of theinvention, the method being characterized in that it comprises thefollowing steps:

-   -   a) where appropriate, positioning said unit pipe element(s)        relative to said leaktight transverse partitions, said        centralizing templates, and said shaping templates, then    -   b) installing said spacers on said unit pipe elements, or        installing a said solid insulating material against the wall of        said unit pipe element; and    -   c) pressing said containers containing a said phase-change        material against said spacers or against a said solid insulating        material; and    -   d) inserting the assembly as obtained in step c) in a said outer        case; and    -   e) where appropriate, injecting a said main insulating material        into the space between said containers and the outer case, and        where appropriate into the space between said containers and the        walls of said unit pipe element(s).

In a preferred implementation of said assembly method, said maininsulating material is a mixture comprising various components which aremixed together and then injected in the liquid state into the variouscompartments defined by said two successive leaktight partitions andsaid insulating material becomes transformed into a gel by at least oneof its said components cross-linking.

This type of gelled matrix has the effect of limiting convection.

Finally, the present invention provides a method of thermally insulatingat least one undersea pipe characterized in that unit thermal insulatingdevices are made as defined above, and the unit thermal insulatingdevices are assembled together end to end as described above.

Other characteristics and advantages of the present invention appearbetter on reading the following description made in non-limitingillustrative manner with reference to the accompanying drawings, inwhich:

FIGS. 1, 2, and 3 are cross-section views of a bundle in the portion ofthe pipes 1 that is surrounded by said containers 3 ₂ in which thecross-section of the case is circular in shape (FIG. 1), rectangularwith rounded corners (FIG. 2), or oval (FIG. 3).

FIGS. 4 and 5 are cross-section views of a bundle of two pipessurrounded by four containers in a manner that is substantiallycontinuous, with spacers 4 (FIG. 4) or shells of syntactic foam 3 ₄(FIG. 5).

FIG. 6 is a side view of a device of the invention in an application toa riser, showing the problems of migration of an insulating materialthat is fluid or semisolid.

FIGS. 7, 8, and 9 are section views showing the cross-section of thedevice at levels a-b-d and e in FIG. 6, respectively for each of thetypes of device, i.e. of circular case (FIG. 7), of oval case (FIG. 8),or of rectangular case with rounded corners (FIG. 9).

FIGS. 10, 11, and 12 show various steps in obtaining an oval case device(FIG. 12) starting from a circular case device (FIG. 10) by impartingdeformation beyond the elastic limit (FIG. 11).

FIGS. 13, 14, and 15 are cross-section views of an oval case devicelevel with a leaktight partition (FIG. 13), and level with acentralizing template (FIGS. 14 and 15), said casing being shown incontraction (FIG. 14) and in expansion (FIG. 15).

FIG. 16 is a side view of a device of the invention presenting aplurality of leaktight partitions, centralizing templates, and shapingtemplates.

FIGS. 17, 18, and 19 are cross-section views of a device of theinvention comprising a case of rectangular shape with rounded cornerslevel with a centralizing template, respectively when at rest (FIG. 17),during expansion of the case (FIG. 18), and during contraction of thecase (FIG. 19).

FIG. 20 is a longitudinal section of a device of the inventionpresenting containers of PCM distributed in the longitudinal directionZZ′ between a leaktight partition, a centralizing template, and ashaping template.

FIG. 21 is a cross-section view of a device of the invention having asingle pipe, shown in section on plane AA′ of FIG. 22.

FIG. 22 is a side view of a device of the invention having a single pipecontained in an outer case held by outer band elements.

1) MAKING CONTAINERS OF PHASE-CHANGE MATERIAL

The containers may be prefabricated at low cost by using materials ofthe polyethylene type, of the polypropylene type, or of any otherplastics material, with shaping being obtained by injection molding,blow molding, or rotary molding. Wall thickness depends on the strengthof the basic material used; in general it is a few millimeters so as toensure sufficient mechanical strength for allowing said containers to behandled after they have been completely filled and finally sealed, whilestill having sufficient flexibility to accommodate variations ininternal volume by deforming.

For a PCM whose chemical composition is incompatible with the maininsulating material, for example a PCM based on metallic salts with amain insulating material based on a gel, the container is advantageouslymade from a sheet of metal which does not react chemically with saidPCM, for example a stainless steel, which sheet having a thickness of afew tenths of a millimeter being shaped, for example by stamping, andwith peripheral sealing being provided by continuous leaktightperipheral welding. The container is then completely filled with PCM andsealed in definitive manner by welding a plug to its filler orifice.

2) DISPOSING CONTAINERS OF PCM AROUND THE PIPES OF A BUNDLE COMPRISINGTWO PIPES

In FIGS. 1 to 5 and 20, there can be seen a thermally insulating devicein cross-section (FIGS. 1 to 5) or in longitudinal section on the axiallongitudinal direction ZZ′ (FIG. 20) of a device for thermallyinsulating two undersea pipes 1 comprising the following:

-   -   an insulating covering comprising containers 3 ₂ of phase-change        material (PCM) 3 ₁ surrounding the set of pipes or each of said        pipes 1, said containers 3 ₂ of PCM 3 ₁ themselves being        surrounded by a main insulating material 3 ₃;    -   said insulating covering being itself covered by a leakproof        protective case 2; and    -   said case being of tubular shape, presenting a longitudinal axis        of symmetry ZZ′, with the cross-section of said case 2 defining        a perimeter presenting two axes of symmetry XX′ and YY′ that are        perpendicular to each other and to said longitudinal axis ZZ′.

The main insulating material 3 ₃ is a material that is subject tomigration, as described below.

In FIGS. 2 and 3, each of said pipes 1 is surrounded by a singlecontainer 3 ₂ such that in cross-section through said pipe level withsaid container(s), each of said pipes is completely surrounded insubstantially continuous manner by said container(s). The only residualgaps 3 ₅ between said containers being provided to make it possible,where appropriate, to inject a main insulating material 3 ₃ between thepipes 1 and said containers, as described below.

In FIGS. 1, 4, and 5, there can be seen a cross-section through a bundleof two pipes, the two pipes being surrounded together by a setrespectively of two containers or of four containers that aredistributed and assembled around the set of two pipes in a manner thatis substantially continuous.

In FIGS. 2 and 3, each of the pipes is surrounded by a single respectivecontainer that is folded over onto itself in such a manner that itslongitudinal edges face each other, preferably close together so as toform a C-shape. Thus, during variations in the volume of the container,the container can deform and the C-shape can tend to open.

In FIG. 1, the outer case 2 is circular and made of a flexiblethermoplastic material.

In FIGS. 2, 4, and 5, the outer case 2 presents a cross-section in theform of a rectangle with rounded corners, while in FIG. 3, it has theshape of an ellipse.

In FIGS. 2 to 5, the outer case 2 may be made of steel sheet or ofcomposite material.

FIGS. 1 to 5 also show, inside the bundle, an auxiliary heater pipe 15and a pipe for injecting water, or an electric cable 16.

In FIG. 4, the containers 3 ₂ are disposed on spacers 4 resting on anddistributed around the pipes 1, the spacers being shown only in theright-hand half of FIG. 4. Said spacers serve to leave a gap between thecontainers 3 ₂ and the walls of the pipes 1, which gap is severalcentimeters across, for example 3 cm. The gap created in this waybetween the containers 3 ₂ and the pipes 1 thus provide insulation,making it possible to avoid transferring too quickly the heat that comesfrom the fluid flowing in the pipes 1 and that flows into the PCM whilethe installation is being restarted after production has been stopped.Advantageously, this is done by filling the gap between the containers 3₂ and the pipe 1 with an insulating material which can be constituted,as in FIG. 4, by the main insulating material 3 ₃ as described below,i.e. a substance of the gel type which is injected in liquid form priorto setting in situ.

FIG. 5 shows another embodiment on the left-hand side in which thecontainers 3 ₂ rest directly on shells 3 ₄ of syntactic foam placeddirectly against the walls of the pipe 1. On the right-hand side of FIG.5, the shells of syntactic foam rest directly on said pipe, while saidcontainers 3 ₂ rest on spacers 4. The gap between the pipe and the twoshells of syntactic foam 3 ₄ is thus more easily filled with the maininsulating material 3 ₃, with this filling operation being made possibleby injecting said insulating material into openings situated in thelongitudinal direction ZZ′ and not shown in the cross-section of FIG. 5,as examined below.

The thin shells of syntactic foam may be several millimeters thick, e.g.12 mm thick, constituting additional high performance insulation thatwithstands pressure.

FIGS. 4 and 5 show a bundle that is designed to rest on the sea bottom10. The coefficient of heat exchange between the bundle and the outsideis smaller through its underface zone directly in contact with the seabed, since the sea bed is more insulating than the flowing water whichis generally at 4° C. and which covers the top and side portions of thebundle which are in contact with the water. That is why, as shown inFIGS. 4 and 5, the thickness of the PCM 3 ₁ included in the containerunderneath the pipes 1 can be less than the thickness of the othercontainers 3 ₂ surrounding the sides and the tops of the pipes 1. Thisadvantageously optimizes the distribution of PCM around the pipes as afunction of heat-transfer requirements.

3) DISTRIBUTION OF PCM CONTAINERS BETWEEN LEAKTIGHT PARTITIONS DISPOSEDALONG THE LONGITUDINAL DIRECTION ZZ′

FIGS. 6, 16, and 20 are longitudinal section views on the axis ZZ′showing a device for thermally insulating two undersea pipes 1, saiddevice comprising:

-   -   a thermally insulating covering 3 ₁-3 ₃ surrounding said        pipe(s);    -   said covering itself being covered by a leakproof outer        protective case 2, and said case 2 being made of a material that        is flexible or semirigid, that is suitable for remaining in        contact with the outside surface of said insulating covering 3        as it deforms; and    -   said case itself being tubular in shape presenting a        longitudinal axis of symmetry ZZ′, the cross-section of said        case defining a perimeter presenting two axes of symmetry XX′        and YY′ that are perpendicular to each other and to said        longitudinal axis ZZ′.

In the longitudinal section views on the axis ZZ′, there can be seenleaktight transverse partitions 5, each of said partitions beingconstituted by a closed rigid structure through which the pipe(s)pass(es), said structure being secured to said pipe(s) and to said case,and said containers are disposed around said pipe(s) between two of saidtransverse partitions.

FIG. 6 is a side view of a portion of a thermally insulated riser 11comprising an outer case 2 made either of thermoplastic material, or ofsteel, or indeed of composite material, of section that may be circularas shown in FIG. 7, or elliptical as shown in FIG. 8, or indeedrectangular with rounded corners as shown in FIG. 9.

Said portion of riser 1 ₁ has two pipes 1 installed near its center, andat its top end (a) and its bottom end (b) it has respective leaktightpartitions 5 serving to co-operate with the outer case 2 to confine theinsulating material 3. Said leaktight partitions 5 support said pipes 1and hold them at a fixed distance apart from each other and at a fixeddistance from the case.

FIGS. 10, 11, and 12 show the section of the outer steel case of abundle, respectively when it is circular in shape on being manufactured(FIG. 10), subsequently deformed beyond its elastic limit in a press byapplication of a force F (FIG. 11), and then constituting, after theforce F has been released, an outer case for a flat bundle, the casebeing substantially oval in section (FIG. 12).

FIGS. 6 and 16 show a portion or segment of a thermal insulation device1 ₁ referred to above as a unit thermal insulation device, comprisingtwo leaktight partitions 5, and a plurality of centralizing templates 6or shaping templates 7. The leaktight partitions 5, the centralizingtemplates 6, and the shaping templates 7 are rigid parts of cylindricalshape as shown in FIG. 20.

In FIGS. 7, 8, 9, and 13, the rigid part constituting a leaktightpartition presents a cross-section having the same shape as thecross-section of the case. The leaktight partition 5 has the pipes 1passing through it. The connection between the leaktight partition andthe pipes 1 is itself leaktight, thus making it possible to confine theinsulating material without leakage within the case. The leaktightpartition 5 is of sufficient strength to freeze the section of the outercase level therewith (levels a and e in FIG. 6). The cylindrical outsidesurface 5 ₁ of the rigid part constituting the leaktight partition 5 isbonded or welded to the case, and advantageously it can also be bandedby an outer hoop 18 placed on the outside of the case level therewith.

Said leaktight partitions 5 are structures that are distinct from saidcase 2, which case presents continuity in the longitudinal directionbetween two points situated on either side of said partition.

In the space confined between two leaktight partitions 5 of said segment1 ₁, there are disposed both centralizing templates 6 and shapingtemplates 7.

The centralizing templates 6 are preferably disposed at regularintervals, for example at a distance apart of 2 m to 5 m. They arelikewise constituted by respective rigid parts secured to the internalpipes 1, with the shape of the centralizing templates allowing the case2 a limited amount of displacement in contraction and in expansion inregister with said centralizing template 6, and more particularly saiddisplacement of the case 2 in register with such a centralizing template6 represents variation lying in the range 0.1% to 10%, and preferably inthe range 0.1% to 5% of the distance between two opposite points 2 ₁-2₂, 2 ₃-2 ₄ of the perimeter of the cross-section of said case.

In FIGS. 17 to 19, said cross-section of said centralizing template 6 isin the form of a rectangle whose corners have been truncated by flatcuts 9 ₁.

In FIGS. 14 and 15 and 17 to 19, the centralizing template 6 presents across-section of perimeter that is inscribed inside a first geometricalFIG. 6 ₁ having the shape of a rectangle with rounded angles (FIG. 17),an oval shape (FIG. 14), and substantially geometrically similar inshape to the geometrical figure defined by the perimeter of thecross-section of the leaktight partition with which it co-operates.

FIGS. 18 and 19 show that the perimeter of the cross-section of thecentralizing template 6 is set back from that of the leaktight partitionand thus from that of the case at rest (FIG. 13), and that it limitsdeformation of said case (FIGS. 18 and 19) by coming into directmechanical abutment thereagainst at at least two opposite points 2 ₁-2₂, 2 ₃-2 ₄ of the perimeter of the cross-section of said case.

In FIG. 20, containers 3 ₂ are shown that extend in the longitudinaldirection ZZ′ between a leaktight partition 5 and a centralizingtemplate 6, and others between a centralizing template 6 and a shapingtemplate 7.

The containers 3 ₂ are filled with a phase-change material 3 ₁ and theyare surrounded by a main insulating material 3 ₂ that is fluid or pasty,and thus subject to migration.

In the event of the inside of the case being at a pressure higher thanthe outside (see zone b in FIGS. 6 and 18), if the case 2 is notcircular but presents a shape that is flattened, in particular oval orrectangular having a major axis of symmetry XX′ and a minor axis ofsymmetry YY′ that are perpendicular to each other and situated in theplane of the cross-section, then the perimeter of the cross-section ofthe case will tend to become more circular. Under such conditions,mechanical abutment and thus contact between the case and saidcentralizing template takes place solely at opposite points 2 ₁, 2 ₂ ofsaid case situated on the major axis of symmetry XX′, such thatexpansion of said case along the minor axis of symmetry YY′ is alsorestricted (see zone b in FIGS. 6 and 18) so the cross-section of saidcase can thus remain flat in shape (i.e. not circular).

In contrast, in the event of the pressure outside the case being greaterthan the pressure inside the case, then mechanical abutment and contactbetween the case and said centralizing template takes placesimultaneously at the opposite points 2 ₁, 2 ₂ of said case situated onthe major axis of symmetry XX′, and at the opposite points 2 ₃, 2 ₄ ofsaid case situated on the minor axis of symmetry YY′ in the plane of thecross-section, such that said contraction of said case is restricted(see zone d in FIGS. 6 and 19). This avoids the case imploding at thesepoints.

At its periphery, mainly in the zones where it makes contact with thecase, the centralizing template 6 advantageously presents a contact area6 ₄ that is sufficiently broad to avoid damaging the outer case 2 whenthe bundle “breathes”. In contrast, the central portion of thecentralizing template can be hollowed out, as shown at 6 ₅ in FIG. 20,so as to minimize thermal bridges while retaining sufficient material toensure that the centralizing template remains sufficiently rigid.

The shaping templates 7 are rigid cylindrical parts of smaller thicknesshaving, like the leaktight partitions 5, the function of fixing theshape of the cross-section of the case at their level, which section ispreferably identical to that imposed by the leaktight partition 5. Thecontact area between the peripheral surface of the cylinder constitutingthe part is thus of smaller height than for the centralizing templatesor the leaktight partitions, as can be seen in FIG. 20, so as tominimize thermal bridges, but like the leaktight partitions, they aresecured to the case by adhesive or welding, and preferably by banding bya hoop 18 on the outside of the case.

The shaping template 7 is not leaktight since it has openings 7 ₁ whichallow the insulating material to pass through, particularly whilefilling with the main insulating material 3 ₃, which material is fluidor semifluid and preferably presents very low viscosity. The shapingtemplate 7 is secured to the internal pipes 1 and maintains a fixeddistance between them as do the leaktight partitions 5 and thecentralizing templates 6. The shaping template 7 also keeps the internalpipes 1 at fixed distances from the case 2 level with the shapingtemplate 7.

Successive shaping templates 7 are preferably spaced apart at a distancelying in the range 5 m to 50 m, and more preferably in the range 5 m to20 m, the outside surfaces of said rigid cylindrical parts constitutingsaid leaktight partition 5 or said shaping template 7 being continuouslyin contact with said case 2. It should be understood that the term“continuously” contact means that said contact is made over the entirecircumference of the periphery of the cross-section of said case.

In FIGS. 7, 8, and 9, there can be seen for each of the levels a, b, d,and e for each of the circular, oval, or rectangular types of case,respectively, and in considerably exaggerated manner, the deformationsthat are caused by differential pressure between the inside of thedevice and the surrounding medium between two leaktight partitionslocated respectively at levels a and e. The pressure differential actingon the case 2 is due to the difference between the density of theinsulating material and that of sea water, the relative density of theinsulating material generally being around 0.8 to 0.85. Thus, by way ofillustration, if consideration is given to a portion of riser 1 that is100 m long, for a main insulating material 3 ₃ of relative density 0.8,the pressure differential between the top and the bottom will be 0.2MPa, the bottom portion (level d, FIG. 6) of said riser being at reducedpressure while the top portion is at increased pressure. As a result theouter case 2 is deformed in a manner that is substantially comparable ineach of the configurations, as shown in FIGS. 7, 8, and 9. The reducedpressure lower down tends to cause the case to contract, as shownbetween plane e and plane d where the section is at its minimum, afterwhich it increases up to a maximum at b, and then decreases towards thenominal section as imposed by the partition 5. Depending on the type ofbundle, the different kinds of deformation are shown for planes a, b, c,d, and e in FIGS. 7 to 9.

These deformations of the outer case cause the semifluid or pastyinsulating material constituting said main insulating material 3 ₃ to betransferred upwardly, possibly also together with said insulatingphase-change material 3 ₁ contained in said container 3 ₂ with thisrunning the risk of impeding proper operation of the insulation, andpossibly even destroying the physical structure, since these types ofinsulating material that are subject to migration remain fragile andremain poor at withstanding the internal shear that is created by suchinternal migration.

In FIG. 7, the case 2 is circular being made of thermoplastic material,and the expansion at level b is large, whereas the contraction observedat level d remains very small since the pressure drop has less effect onits final shape.

This phenomenon of the fluid being transferred upwardly is in additionto the above-described “breathing” phenomenon due to variations in thetemperature of the internal pipes which lead to variations in volume,mainly within the insulating mixture, which have the effect ofamplifying deformation, particularly in the upper portion.

When a said insulating material is transferred by migration along saidlongitudinal direction ZZ′, generally in the upward direction when saidpipe is a riser or on a slope, the shape of the perimeter of thecross-section of said case 2 is not uniform along the longitudinal axisof symmetry ZZ′, and said fixed shape of the perimeter of thecross-section of said leaktight partition 5 then corresponds to thecross-section of said case prior to said transfer of material, i.e.while the insulating covering 3 ₁-3 ₃ is uniformly distributed aroundsaid pipe along said longitudinal axis ZZ′, and said shape of thecross-section of the case along said longitudinal axis is likewiseuniform.

When deformation of the case 2 occurs and the shape of the outsidesurface of the insulating covering 3 ₁-3 ₃ becomes deformed, then theshape of the perimeter of said cross-section of the case remainsgenerally symmetrical relative to said mutually perpendicular axes XX′and YY′ that are also perpendicular to said longitudinal axis ofsymmetry ZZ′ of the case.

The leaktight partitions, the centralizing templates 6, and the shapingtemplates 7 are preferably made of strong materials that are poorconductors of heat, for example optionally reinforced thermoplasticmaterials or composite materials or even partially out of metal, andadvantageously as a combination of these various technologies.

The greatest diameters of the leaktight partitions, the centralizingtemplates, and the shaping templates in their cross-sections are of theorder of 1 m to 1.5 m, or even 2 m, depending on the overall size of thedevices of the invention and also corresponding to case thicknesses ofabout 15 mm to 40 mm for cases that are flexible being made ofpolyethylene and polypropylene the thermoplastic materials, and about 5mm to 8 mm for semirigid cases made of steel or composite material, andfor pipes having diameters of the order of 100 mm to 400 mm.

4) DISTRIBUTION OF CONTAINERS IN AN INSULATING DEVICE HAVING A SINGLEPIPE AND NOT HAVING ANY CENTRALIZING TEMPLATE OR SHAPING TEMPLATE

FIGS. 21 and 22 show a thermally insulating device of the inventioncontaining a single undersea pipe 1 and not containing any shaping orcentralizing templates insofar as the outer flexible case 2 is held inposition by banding elements 11 which prevent excessive deformation ofthe case during any migration of said main insulating material 3 ₃ andsaid phase-change material 3 ₁ contained in the containers 3 ₂.

In FIGS. 21 and 22, there can be seen a device of the inventioncomprising an insulated pipe contained in a polyethylene type flexibleouter case 2. The pipe is covered in a first insulation systemconstituted by two half-shells 3 ₄ of syntactic foam stuck to theoutside wall of said pipe 1. These shells of syntactic foam aresurrounded by two containers 3 ₂ likewise in the form of half-shells andcontaining a PCM 3 ₁, These containers 3 ₂ are themselves held at adistance from shells of syntactic foam 3 ₄ by spacers 4 stuck to saidshells of syntactic foam.

The containers 3 ₂ are held on the syntactic foam shells 3 ₄ by straps(not shown).

As shown in the longitudinal view of FIG. 22 the containers 3 ₂ are notincorporated between centralizing templates or shaping templates. Inthis case, the outer case 2 is secured to said pipe 1 via transversepartitions 5 (not shown) that are optionally leaktight, and that aresituated in register with the banding elements 11 on the outside of thecase 2.

The outer case is completely filled with a main insulating material 3 ₃,and the gaps between the containers 3 ₂ and the syntactic shells 3 ₄ arethere to make it easier to fill completely all of the residual spaceinside the case 2. Since the outer case 2 is deformable, it can deformto take up a curved shape 12 without significantly increasing itsinternal pressure compared with the pressure at the sea bottom, saidcurve 12 serving to absorb phenomena associated with variation in theinternal volume 13 of the containers 3 ₂ which variation can be positiveor negative.

As shown in FIGS. 21 and 22, it can be seen that the containers are alsodistributed and spaced apart substantially continuously along thelongitudinal direction ZZ′ around said pipe 1 in such a manner as alsoto avoid creating cold points between two consecutive containersdisposed along the longitudinal direction ZZ′.

5) METHOD OF ASSEMBLING A DEVICE FOR THERMALLY INSULATING A PIPE, ANDINSTALLING SAID PIPE AT SEA

Unit segments 1 can be assembled together to form a continuous bundlehaving a length of several kilometers in the following manner, forexample. A first segment is made initially, e.g. of length 100 m, asshown in FIG. 16 and it is fitted with its leaktight partitions. Forthis purpose, the internal pipes are made from 12 m-long pieces of tubethat are joined together end to end by welding. Thereafter, thecentralizing templates 6 and the shaping templates 7 are installedprogressively, said centralizing and shaping templates being fitted, forexample, with bottom wheels or skids. Where appropriate, the spacers 4are installed on said pipes 1 between the centralizing templates 6 andthe shaping templates 7, and containers 3 ₂ are pressed against saidspacers 4 by means of straps (not shown).

The assembly is then inserted inside the outer case 2 by pushing theprefabricated assembly as a single piece or by joining end to end caseunits that are 12 m or 24 m, or longer, with said end-to-end joiningbeing preferably performed at a distance from the ends of the insidepipes. Once the outer case is in place, said leaktight partitions 5 areinstalled at each end and the case 2 and the pipes 1 are connectedthereto, after which the main insulating material 3 ₃ is injectedbetween the leaktight partitions into the space that remains between thepipes 1 and the containers 3 ₂, and the space that remains between thecontainers 3 ₂ and the case 2. Care is taken to allow the end of theouter case that is to be extended to project by a certain length, e.g.by 20 cm, and also to allow the internal pipes 1 to project by a certainamount, e.g. the first pipe by 1 m and the second pipe 1.5 m, such thatthe zones in which the welded joints are to be made are offset from oneanother, thereby allowing access for welding equipment and forinspection equipment. The prefabricated segment is then towed towardsthe sea so as to release the working zone, and the following segment ismade in similar manner. When the outer case 2 is put into place aroundthis new segment, it is brought face to face with the case that hasalready been made at one end of the preceding segment, and the ends arewelded together. At the other end, a leaktight partition 5 is installedwhich is secured to said outer case and to the pipes, after which thecase is filled with insulating material. Care is taken to allow thepipes 1 to project by different lengths from the outer case as describedabove so as to facilitate welding and inspection operations, and theoperation is repeated until a sufficient total length has been obtained.

If electrical cables or umbilical cables are to be installed within thedevice, an additional pipe is provided to act as a sheath for the cable,and the cable is drawn along said sheath only after the complete lengthof the bundle has been assembled, which length may be as much as severalkilometers.

Once the entire line has been made, e.g. to a length of 2500 m, it ispulled to the sea, and towed to the site where it is to be installed,which site may be several hundreds of kilometers away. Thereafter, it iseither put into place on the bottom, or else it is stood up in avertical position so as to be secured to an anchoring base, for a hybridriser tower, or indeed so as to be installed in a simple catenaryconfiguration suspended from an anchored support floating on thesurface.

6) INJECTING A MAIN INSULATING MATERIAL 3 ₃ INTO THE INSIDE OF THE CASE

In a preferred version of the invention, the main insulating material 3₃ is advantageously made as a cross-linked gel that presents a highdegree of stability, for example a compound of the polyurethane type orof the silicone type, which on cross-linking creates a gel that issubstantially continuous, and which acts as a matrix containingdispersed therein a liquid such as a paraffin, gas oil, or any othercompound presenting a low level of thermal conductivity. While it isbeing made, it is advantageous also to incorporate solid compounds inthe main insulating material 3 ₃, for example microspheres of glass,which have the function of reducing the thermal conductivity of themixture, or fiber matrices whose function is to reduce the convection ofparticles remaining in the liquid state within said main insulatingmaterial 3 ₃.

The various components are mixed together and then blended energeticallyso as to obtain a uniform slurry which can then be injected in liquidform and thus fill all of the empty residual space in the segment thatis defined between two consecutive leaktight partitions. Prior toinjecting the fluid main insulating material 3 ₃, the segment isadvantageously evacuated so as to avoid leaving any residual pockets ofgas. The vacuum created in this way is certain to cause the case toimplode locally, but the case will return to its initial shape as soonas the necessary and sufficient quantity of fluid has been injected.Precautions are naturally taken to ensure that the shaping andcentralizing templates 7 and 6 are dimensioned and spaced apartappropriately so that such temporary implosion has no significantrepercussions on the integrity of the outer case 2.

The homogenized fluid injected in this way is in the liquid state whilefilling is taking place, but once the binder has cross-linked ittransforms into a gelled matrix within which the other component(s)is/are held captive, which component(s) remain(s) in the liquid state orlikewise become(s) a gel, thereby greatly reducing convection phenomena.

The binder components, e.g. polyurethanes or silicone-based compounds,are preferably selected in such a manner that polymerization begins onlyafter several hours, for example after a minimum duration of 6 hours to8 hours, thus making it possible with reasonable mixing and pumpingmeans to make insulating devices having a diameter of 1 m and a unitsegment length of about 100 m within the available time lapse.

The unit length could also be shortened or lengthened by using injectionmeans of smaller or greater capacity, or indeed by using components,optionally including retarders, whose workability time is shorter orlonger, the essential point being that the entire injection operationmust be terminated before gelling or cross-linking reaction has beeninitiated to any significant extent.

By operating in this way, the filling of the bundle is considerablysimplified, since the complicated tasks needed in the prior art forinstalling absorbent matrices and causing the insulating fluid topercolate therethrough are avoided, as indeed are operations thatconsist in injecting an insulating mixture such as paraffin while it ishot and that is liable to shrink considerably on passing from the liquidstate to the solid state.

1. A device for thermally insulating at least one undersea pipe, the device comprising: a thermally insulating covering surrounding said pipe; said covering itself being covered by an outer leakproof protective case, and said case being made of a flexible or semirigid material suitable for remaining in contact with the outside surface of said insulating covering when it deforms, the device being characterized in that: said insulating covering comprises a phase-change material confined in at least one container made of a flexible or semirigid material that is deformable; and said container being disposed around said pipe.
 2. An insulating device according to claim 1, characterized in that in a cross-section of said pipe, level with said container, said pipe is surrounded by said container in a substantially continuous manner.
 3. An insulating device according to claim 1, characterized in that said container is placed close to the pipe in such a manner that said pipe does not come directly into contact with said container.
 4. A device according to claim 3, characterized in that said container comprises a plurality of containers which are disposed against spacers, said spacers being disposed against and around said pipe in such a manner as to leave a gap between said containers and said pipe.
 5. A device according to claim 4, characterized in that said containers are spaced apart from said pipe by a distance of 5 mm to 10 cm, and preferably by a distance of 1 cm to 5 cm.
 6. An insulating device according to any preceding claim, characterized in that said pipe is surrounded by a second insulating material that is solid, being applied against said pipe, said container being pressed against said solid insulating material surrounding said pipe.
 7. A device according to claim 1, characterized in that said insulating covering covered in a said leakproof protective case comprises a main insulating material and said container of phase-change material surrounding said pipe.
 8. An insulating device according to claim 7, characterized in that said main insulating material surrounds said pipe and provides separation between said pipe and said container in the gap between said container and said pipe.
 9. An insulating device according to claim 1, wherein said pipe comprises a plurality of pipes and wherein said container comprises a plurality of containers characterized in that in the portions of the pipe(s) surrounded by said containers, the device has at least two and preferably three or four containers in a said cross-section of said pipe(s) surrounded by said containers, and also preferably surrounding said pipe(s) in a manner that is substantially continuous.
 10. An insulating device according to claim 1, characterized in that said phase-change material presents a liquid/solid melting temperature that preferably lies in the range 20° C. to 80° C., that is lower than the temperature of the fluid flowing in said pipe when it is in operation, and higher than the temperature at which the fluid flowing inside the pipe present an increase in viscosity that is harmful for its ability to flow in said pipe.
 11. A device according to claim 10, characterized in that said insulating phase-change material comprises chemical compounds of the alkane family, preferably a paraffin having a hydrocarbon chain with at least 14 carbon atoms.
 12. A device according to claim 11, characterized in that said paraffin is heptacosane of formula C₁₇H₃₆ presenting a melting temperature of about 50° C.
 13. A device according to claim 1, characterized in that said main insulating material is constituted by an insulating mixture comprising a first compound consisting in a hydrocarbon compound such as paraffin or gas oil, mixed with a second compound consisting in a gelling compound and/or a structuring effect compound, in particular by means of cross-linking, such as a second compound of the polyurethane type, of the cross-linked polypropylene type, of the cross-linked polyethylene type, or of the silicone type, and preferably said first compound is in the form of particles or microcapsules dispersed within a matrix of said second compound.
 14. A device according to claim 13, characterized in that said first compound is selected from alkanes such as paraffins, waxes, bitumens, tars, fatty alcohols, and glycols, said first compound preferably being a phase-change compound.
 15. A device for thermally insulating at least one undersea pipe, the device being characterized in that it includes at least two leaktight transverse partitions, each of said partitions being constituted by a closed rigid structure having said pipe passing therethrough, and secured to said pipe and to said case, and said containers being disposed around said pipe between said two transverse partitions.
 16. A device according to claim 15, characterized in that said transverse partitions are spaced apart, preferably at regular intervals, along said longitudinal axis by a distance of 50 m to 200 m.
 17. A device according to claim 15, characterized in that it includes at least one centralizing template, preferably a plurality of centralizing templates located, preferably at regular intervals, between said two successive leaktight transverse partitions along said longitudinal axis, each centralizing template being constituted by a rigid part secured to said pipe(s) and presenting a shape which allows limited displacement of said case in contraction and in expansion in register with said centralizing template, said containers being disposed between two successive ones of said centralizing templates, where appropriate.
 18. A device according to claim 17, characterized in that said centralizing template is constituted by a rigid part, preferably having a cylindrical outside surface with a cross-section whose perimeter is set back relative to that of said leaktight partition, the centralizing template limiting deformation of said case by the case coming into mechanical abutment against said rigid part at at least two opposite points of the perimeter of the cross-section of said case, said displacement of the case in register with a said centralizing template representing variation of 0.1% to 10%, and preferably of 0.1% to 5%, of the distance between two opposite points of the perimeter of the cross-section of said case.
 19. A device according to claim 17, characterized in that said rigid piece constituting said centralizing template presents a portion of its outside surface that is set back sufficiently relative to the surface of the case, and/or presents perorations passing through it, so as to create a space allowing the material constituting said insulating covering to be transferred through said centralizing template.
 20. A device according to claim 16, characterized in that it has a plurality of said centralizing templates, and two successive centralizing templates are spaced apart along said longitudinal axis of the case by a distance of 2 m to 5 m, with said containers being interposed between two successive ones of said centralizing templates.
 21. A device according to claim 16, characterized in that it has at least one, and preferably a plurality of shaping templates each constituted by a rigid structure secured to said pipe(s) with the pipe(s) passing therethrough, and secured at its periphery to said case, the shaping template(s) being disposed between two successive ones of said leaktight partitions, said shaping template having openings allowing the material constituting said main insulating material to pass through said shaping template.
 22. A device according to claim 21, characterized in that said open structure of said shaping template is constituted by a cylindrical part presenting a cross-section of perimeter that is inscribed in a geometrical figure identical to the geometrical figure defined by the shape of the perimeter of the cross-section of said leaktight partition.
 23. A device according to claim 21, characterized in that it has a plurality of shaping templates disposed along said longitudinal axis of the case, preferably at regular intervals, two successive shaping templates being preferably spaced apart by 20 m to 50 m.
 24. A device according to claim 1, characterized in that said case defines a perimeter presenting two axes of symmetry that are perpendicular to each other and to said longitudinal axis.
 25. A device according to claim 24, characterized in that said cross-section of the case is circular in shape.
 26. A device according to claim 24, characterized in that said cross-section of the case is oval in shape.
 27. A device according to claim 24, characterized in that said cross-section of the case is rectangular in shape, preferably with rounded corners.
 28. A device for thermally insulating a bundle of undersea pipes, the device being characterized in that it comprises a device according to claim 1 having at least two of said undersea pipes disposed in parallel.
 29. A device according to claim 28, characterized in that said leaktight partitions, said centralizing templates, and said shaping templates hold at least two of said undersea pipes at a fixed distance apart.
 30. A unit thermally insulating device suitable for building a device according to claim 1 by assembling said unit thermally insulating devices end to end, the unit device being characterized in that it comprises: one or more unit undersea pipe elements replacing the undersea pipe(s); and an insulating covering, a said protective case, and a said insulating covering comprising at least one said container containing a said insulating phase-change material as defined in claims 1 to 14, each said unit element having at at least one of its ends or at both ends, a said leaktight partition, and preferably said centralizing templates and also preferably shaping templates as defined in claims 15 to 29 disposed between two successive leaktight partitions.
 31. A method of assembling a unit device according to claim 30, characterized in that it comprises the following steps: a) where appropriate, positioning said unit pipe element(s) relative to said leaktight transverse partitions, said centralizing templates, and said shaping templates, then b) installing said spacers on said unit pipe elements, or installing a said solid insulating material (3 ₂) against the wall of said unit pipe element; and c) pressing said containers containing a said insulating phase-change material against said spacers or against a said solid insulating material; and d) inserting the assembly as obtained in step c) in a said outer case; and e) where appropriate, injecting a said main insulating material into the space between said containers and the outer case, and where appropriate into the space between said containers and the walls of said unit pipe element(s).
 32. A method according to claim 31, characterized in that said main insulating material is a mixture comprising various components which are mixed together and then injected in the liquid state into the various compartments defined by said two successive leaktight partitions and said insulating material becomes transformed into a gel by at least one of its said components cross-linking.
 33. A method of thermally insulating at least one undersea pipe, the method being characterized in that unit thermally insulating devices according to claim 30 are made and then assembled together end to end. 